The removal of heat from devices mounted to a printed circuit board (PCB) has presented ongoing challenges for designers, especially in the design of cards which function as power supply boards, since device reliability decreases as device temperature increases. Devices used for power conversion, such as power diodes and power switching devices, tend to require relatively effective heatsinks to achieve adequate function and reliability. A traditional approach is to attach pin through-hole (PTH) power devices by any of several means to a heat sink, and to place this assembly onto the PCB for wave soldering. This approach requires a significant amount of hand assembly plus a wave soldering process (for PTH part attachment to the PCB), typically in addition to an existing reflow process if surface mount components are used. Such a process may tend to be relatively costly.
An alternative process has been to use surface mount (SMT) power devices which eliminate the wave soldering process and which permit automation of the assembly operation. This approach is limited by the ability of the PCB copper planes to remove heat from the various power devices.
It is also possible to bond the underside of the casing of a power device directly to the surface of the circuit board. A copper layer can be exposed under the device, so that the copper is effectively bonded to the casing. The copper may be conductively linked to adjacent pads, and heatsinks can be soldered to the adjacent pads. The heat is thus able to flow from the device into the copper plane, and thence along the board to the heatsink whence it is carried away by convection inside the electronic device enclosure. There may be two disadvantages to this approach. First, the heat conduction is limited to the cross-sectional area of the copper path between the device and the adjacent heatsink. The width of that path is limited, since not all of the width of the board can be devoted to carrying heat. In the other cross-sectional dimension, the thickness of the copper plane is very limited, a 2 oz., standard thickness layer being only 0.0028" thick. Second, the heat must cross not one, but two solder paste, or conductive, adhesive interfaces. These interfaces are not always good conductors, and are not always well and uniformly made. It would be advantageous to be able to use a much thicker piece of conductive material, whether copper, aluminium or some other medium, and to reduce the number of adhesive interruptions in the heat conduction path.
In the case of a surface mounted device, it may be desirable to provide a heatsink that permits the device to locate flush with the printed circuit board, as if it were sitting on the PCB rather than on top of a heatsink. This may be advantageous where the device has a pre-defined footprint that assumes a flush mounting. It is also of some importance that a pick-and-place machine be able to manipulate the heatsink relatively easily, and that the heatsink and the device to be mounted on it both be in a configuration to accept soldering in an oven.
It often occurs that relatively high heat generation devices, such as transformers with ferro-magnetic cores, are also relatively heavy, as are some heatsinks. This may lead to shock and vibration challenges in a card and rack assembly. In that light, it would be helpful in those circumstances to adopt a type of heatsink that can also assist in the structural stabilization of the printed circuit board. Alternatively, or additionally, it could be helpful for the heat sink to double as a path for the conduction of heat and as an electrically conductive power or ground plane of the printed circuit board.
A power supply card may employ a single turn winding about a core to yield a relatively high current, low voltage signal. In such instances it may be desirable to provide a conductor of large cross-sectional area. It may be preferable to employ a surface mounted monolith of formed copper sheet, for example, to yield the desired path. Use of a conductive surface mounted part also requires care in maintaining creepage distances between the conductive secondary element and neighbouring primary elements on the board. A creepage distance is the isolation distance along a surface between primary and secondary circuitry. The permissible vertical clearance distance is 4 mm, whereas the permissible creepage distance along the surface of a card is 8 mm, as required to meet the reinforced isolation requirement of UL950 for 400 vdc primary voltage. In some instances then, lifting the copper from the board surface may permit a tighter clearance distance to be used, and may tend not to occupy as large a footprint on the board itself